This invention relates generally to high fidelity loudspeakers, and more particularly to a type thereof wherein a vibrating plate or diaphragm driven by a moving coil type driver unit is coupled through an acoustic transformer to a tubular member of a substantial length, whereby a suitable acoustic radiation resistance is imparted to the diaphragm to place it in resistance control condition thereby to make possible the high fidelity acoustic reproduction over a wide sound range especially for a low sound range of 20 c/s to several hundreds of c/s.
As a result of recent study, it has been found that the direct radiation type loudspeakers such as those having conical diaphragms can produce sounds under inertia control condition such that the amplitude of the diaphragm increases in inverse proportion to the second power of its frequency and, in some cases, exceeds several millimeters when the frequency of the sound is extremely low. Such a great amplitude of the diaphragm inevitably causes free vibrations of the diaphragm, thus giving rise to the generation of creaking noise due to the extreme deformation of the material of the diaphragm, such noise being particularly detrimental to the tone color of the reproduced sound.
Although this kind of noise can be eliminated by the employment of a horn type loudspeaker having a conventional conical or exponential horn, the diaphragm thereof being placed in resistance control condition for a frequency range exceeding the cut-off frequency, the horn type loudspeaker is adapted to the reproduction of high-frequency sound rather than low-frequency sound. Accordingly, when it is desired to reproduce a sound of a frequency lower than the cut-off frequency of the horn type loudspeaker with high fidelity, the peripheral length of the opening portion of the horn must be greater than the wavelength .lambda. of the cut-off frequency (.lambda. = 341 m/sec. .div. 20 c/s .apprxeq. 17.0 m), and the diameter D of the same portion must be greater than .lambda. .div. .pi. = 5.43 m. It is almost impossible to manufacture a loudspeaker of such enormous size.
Furthermore, the acoustic impedance of the conical or exponential horn is not a pure resistance over a low frequency range lower than the cut-off frequency. Therefore, it is difficult for the horn type loud-speaker to produce sound of a frequency lower than the cut-off frequency under resistance control condition.
Thus, the various types of the conventional loudspeakers have difficulties in producing sound of a frequency varying in a wide range with high fidelity under resistance control condition, and these difficulties become worse particularly when the sound is in an extremely low frequency range.